1. Field of the Invention
The present invention concerns a CMOS output stage with a large voltage swing and with stabilization of the quiescent current.
The present invention concerns, in general, semiconductor amplifiers of the integrated, that is monolithic, type and more particularly, integrated amplifiers in complementary MOS (briefly CMOS) type devices; that is, in monolithic devices formed on a single chip of semiconductor material, typically silicon, wherein the active elements (diodes, transistors, etc.) are generally unipolar elements, of the surface field effect type and may be of an N channel or P channel type, channel type although it is possible to also form junction type bipolar active elements on the same chip or substrate of semiconductor material in order to satisfy particular circuit requirements. Furthermore, the invention is particularly effective and useful for making analog subsystems in digital integrated circuits, that is, for implementing analog functions in integrated digital devices.
2. Description of the Prior Art
Lately the necessity (or usefulness) of implementing analog and digital subsystems in the same integrated circuit using the same fabrication technology has become evermore frequent. For this reason, the implementation of analog functions in MOS technology (Metal-Oxide-Semiconductor) assumes an ever greater importance and in particular, the development of operational amplifiers made with surface field effect elements has had greatly increased.
The operational amplifier is, in fact, the key element, a real building block, or the majority of the analog systems and its characteristics determine decisively the characteristics of the whole system itself.
The requirements which have to be met when designing an operational amplifier which must be used within a monolithic analog subsystem are far different from those of traditional "self standing" operational amplifiers made with bipolar elements. The main difference rests in the fact that, for the majority of the operational amplifiers of a subsystem, the load to be driven is already defined and often purely capacitive with values of a few picofarad, while the "self standing" operational amplifiers are for more general applications and thence must offer certain characteristics necessarily independent of the type of load which may be capacitive (up to hundreds of picofarads) and/or resistive (down to a minimum of about 1 K.OMEGA.).
Moreover, inside a monolithic analog subsystem, only very few operational amplifiers must drive a signal outside the chip, that is outside the integrated circuit, where the capacitive and/or resistive loads may take significant values or be quite variable. These amplifiers are called buffer amplifiers, or simply output buffers. The amplifiers, the outputs of which do not leave the integrated circuit, are called instead internal amplifiers. These two types of operational amplifiers are structurally similar in the sense that both are formed by an input differential stage and by a gain stage (FIGS. 1a and 1b) except that, for the separation or output operational amplifiers (buffers) it is necessary to also provide a buffer stage, called an output stage, which permits driving heavy external loads without degrading the gain or the stability of the first two stages (FIG. 1b).
Dynamic characteristics (transient response, bandwidth, settling time) are determined exclusively by the first two stages, that is, the input stage and the gain stage, which together form what is commonly indicated by the term "core"; therefore it is necessary that the output stage have a wide bandwidth with respect to the core and that it introduce a phase shift of only few degrees at the open loop cut-off frequency of the core in order not to degrade the dynamic performances of the whole operational amplifier.
Other requirements of the output stage are a low output impedance (much smaller of that of the load), a large maximum swing of the output signal, that is, a high peak value of the output voltage before clipping begins to take place, and the ability to supply a high current to the load with relatively low total harmonic distortion, that is, a high linearity. These last two characteristics are typical of power stages.
Frequently, a source follower is used as the output stage, that is, a stage formed by two P-channel or N-channel MOS transistors in a common drain configuration, which provides a voltage gain lower than unity and a large current gain (FIG. 2).
An output stage of this type is characterized by an extremely wide bandwidth and introduces a negligible phase shift at the open loop cut-off frequency of the operational amplifier. On the other hand, it presents some important drawbacks; namely:
(a) the voltage excursion on the load, positive in the case of a source follower stage made with N channel devices or negative in the case of a source follower stage made with P-channel devices, are limited by a sum of intrinsic characteristics of the two integrated MOS transistors, that is, by the value of the cut-in threshold voltage, by the body effect, and by the overdriving;
(b) a limited ability of absorbing current from the current generator M2 (FIG. 2).
With the aim of overcoming the first drawback, it has been proposed to use an emitter follower output stage utilizing a junction type bipolar transistor Q1 in place of the MOS transistor (FIG. 3), that is, purposely forming a bipolar transistor of the junction type on the same CMOS chip, which is an already consolidated technique that does not require additional masks beside those normally used by the CMOS process.
However, this solution has a drawback too; namely:
(i) a persistent limited capability of absorbing current from the current generator M2;
(ii) the risk of degrading the gain of the preceding stage (operational amplifier) if .beta..sub.Q1 is not very high because the impedance seen from the base of Q1 approximately equals the product .beta..sub.Q1 R.sub.L, where R.sub.L is the external load, and the possibility of encountering stability problems due to the low and hardly controllable cut-off frequency (f.sub.T) of the bipolar transistor Q1;
(iii) the possibility of establishing a parasitic SCR (latch-up) caused by the collector current flowing through the substrate of the integrated circuit.
Another widely used solution is the class-AB output stage (FIG. 4). The characteristics of this stage are practically very similar to those of the simple source follower stage though not presenting limitations on the capability of absorbing or delivering current, respectively, from or to the load; nevertheless, this output stage presents the drawbacks of an output impedance which is relatively high with respect to the load and of a maximum excursion of the output signal is limited both towards the voltage V.sub.DD as well as towards voltage V.sub.SS.